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EuroWire – July 2007
97
english
Class 10 H is hard drawn material with
10% copper cladding by volume. The
aluminium core is EC grade aluminium.
Copper thickness for this material is
3.5% of the radius of the wire. The com-
bined materials have a specific gravity of
3.32 compared to 8.89 for solid copper.
Cladding thickness of the finished 34 AWG
conductor is only 32 microns. However,
when considering the resistance of the
shield, the resistance of the outer layer
must be taken into account.
This is due to the skin effect. In a cylindrical
conductor, the skin effect causes the
current gradient to concentrate towards
the surface of the conductors. This effect
can be quantified in
formula 1
as shown
above.
While the overall conductivity is 62%
that of copper, the skin effect results
in equivalent resistance at higher
frequencies.
Overview of testing
and results
Screening attenuation measurements were
conducted according to IEC 62153-4-4.
A commercially available triaxial fixture
known as a CoMet tube was used for the
measurements.
Figure 1
shows a screening attenuation
comparison of the copper clad aluminium,
CCA, braided coaxial cable to the standard
copper, Cu, braided shield.
This chart covers the frequency range from
5MHz to 1GHz. The traces are an average
of five measurements on five separate
two-metre samples of each shield design.
Note that the overall curves are very
similar with the CCA braided samples
showing slightly better results than the
standard. Both shields show a sharp
improvement in screening attenuation at
the lower frequencies, dropping into the
-75 dB range.
Figure 2
expands the data from the
higher frequency screening attenuation
measurements. The data from 0.3 to
5 MHz was collected during the
same measurements as the data from
Figure 1
.
Here the data curves begin to merge
at approximately 4.5 MHz and the solid
copper material begins to drop at a slightly
greater rate.
The differences in the curves at both the
high and low frequencies are relatively
small and may fall within the variability of
the tests.
According to
Formula 1
, the skin depth at
4.2 MHz is 21 microns.
4.2 MHz is the highest frequency in
most security video applications. There
appears to be no problem with the
RF current carrying capacity of the
copper clad shielding conductors at the
higher frequencies used in standard NTSC
video waveforms.
However, the low frequency component
of the waveform and the relationship to
the high frequency components need to
be examined.
Composite video testing focused on
parameters that may show any tendency
toward dispersion using the copper clad
aluminium conductors. Test methods
were taken from ANSI Standard T1.502-
2004, System M-NTSC Television Signals
– Network Interface Specifications and
Performance Parameters.
Tests were performed using a Tektronix
TSG95 Video Test Signal Generator driving
the cable under test, which was connected
to a Tektronix VM700 Video Measurement
Test Set.
All
measurements, with the exception
of the multi-burst were taken using an
NTC-7 composite test signal. Test results
are shown for 700 ft (213m) and 500 ft
(152m) lengths of cable. Note that video
signal levels are expressed in IRE units.
Zero IRE in an NTSC video pulse is defined
as the blanking level with 100 IRE equalling
full white.
Formula 1
▲
Table 1
:
Chrominance/Luminance Gain Inequality
▲
Table 2
:
Chrominance/Luminance Delay Inequality
▲
Table 3
:
Line Time Waveform Distortion
▲
Table 4
:
Insertion Gain
▲
RG 59 Type 95% Cu
RG 59 95% CCA
700 feet (213 metres)
73.8
72.8
500 feet (152 metres)
80.4
79.6
T1.502 Specifications (Long Hall) +/- 7 IRE (93% – 107%)
RG 59 Type 95% Cu
RG 59 95% CCA
700 feet (213 metres)
-51
-51
500 feet (152 metres)
-37
-36
T1.502 Specifications (Long Hall) +/- 54 ns
RG 59 Type 95% Cu
RG 59 95% CCA
700 feet (213 metres)
2.0
2.0
500 feet (152 metres)
1.5
1.4
T1.502 Specifications (Long Hall) 1.5 IRE P-P (+/-1.5% P-P)
RG 59 Type 95% Cu
RG 59 95% CCA
700 feet (213 metres)
94.2
93.6
500 feet (152 metres)
96.0
95.3
T1.502 Specifications (Long Hall) +5.9 IRE to -5.5 IRE (+105.9% - 94.5%)